The present invention generally relates to fabrication of heat exchangers and, more particularly, to a method for brazing open cell reticulated foam pieces to stainless steel tubing.
A thermal cycling absorption process (TCAP) is currently used to separate and purify hydrogen isotopes from hydrogen gas. The TCAP process uses differences in absorption properties in palladium to separate the low mass and high mass isotopes. The TCAP unit uses heat exchangers in part of the process to derive product (high mass isotopes) and raffinate (low mass isotopes). It is possible to purify hydrogen isotopes to the tens of parts per million (ppm) range using the TCAP technology. Although the TCAP process has been used successfully for a number of years, there is a possibility for significant improvement in cycle time and thermal efficiency by changing the heating and cooling medium from gaseous nitrogen to a thermal fluid. This change in thermal media requires significant changes in equipment design.
The improved TCAP heat exchanger uses a tube-in-tube (TnT) configuration in which an inner tube is surrounded by an outer tube with an annular space between the tubes. The thermal fluid is contained within the inner tube and the hydrogen gas, which is to be heated and cooled, is circulated in the annular space between the tubes. A vacuum jacket may surround the outer tube. The palladium used for adsorption/desorption fills the annular space. The palladium may be in the form of particles or pellets of palladium coated kieselguhr, as known in the art. The TCAP process cycles between approximate −25 and 125° C. Decreasing the duration of the cycle will increase the throughput of impure gas and increase the productivity of the system. The desire for a rapid change in temperature of the system and an improvement in overall heat transfer may be achieved by adding high thermal conductivity metal to the tube-in-tube TCAP annular space and will improve the efficiency, however, it is also desirable to keep the overall thermal mass as low as possible to maximize the heating and cooling of the product gases, rather then the process hardware.
The addition of copper foam into the annular space could improve the thermal properties of the heat exchanger. The copper foam could be friction fit onto the inner tube or metallurgically bonded to the inner tube for improved heat transfer to the palladium coated kieselguhr particles. Bonding to both inner and outer tubes does not appear feasible, however, due to potential thermal fatigue problems from thermal expansion and contraction differences in which the foam would likely fracture.
The open cell structure of one particular type of copper foam results in a relative density of the foam versus solid copper of about 6.5-9.5%, i.e., less than one-tenth the density of solid copper. Prior art techniques used for bonding solid copper to stainless steel have generally not proved successful for metallurgically bonding copper foam to stainless steel tubing. Typical prior art braze alloy placement techniques may introduce problems such as excessive braze material (with foil), introduction of organic binder materials (with paste) or additional required heating cycles (with other precoats). The excessive braze material produced by prior art brazing techniques may be unsuitable due to problems with braze pooling, braze wrinkling, braze blistering, and brazing erosion, i.e., alloying of the braze with the copper to produce a low melting point alloy that melts the filament resulting in a loss of contract during brazing. This significantly reduces the heat transfer between the inner tube and copper foam, and therefore reduces the heat transfer between the thermal fluid and palladium particles in the TnT application. This may also result in inadequate total bond strength.
For high temperature brazes, (for example, at temperatures above about 800° C.) additional problems are encountered. For example, the outer portion of the copper foam may be crushed inside the outer tube due to thermal coefficient of expansion mismatch as the cooper foam in the annular space expands much more when heated than does the surrounding outer stainless steel tube. Under the high temperature, the copper foam may also be subject to creep deformation, causing the copper foam to separate from the inner surface and lose contact with the tube. Partial melting of the copper foam may occur at brazing temperatures in the range of 980° C. if the copper foam becomes contaminated with other materials (melting point of pure copper is 1085° C.). To be successful, a brazing technique should produce a wetted surface on the tube or structure to which the copper foam is to be bonded, the ligaments of the copper foam should be bonded without brazing erosion of the copper foam, and the braze joints should be stronger than the filaments, i.e., the filaments themselves should break before the braze joints under destructive axial load testing.
As can be seen, there is a need to metallurgically bond copper foam to other metals, such as stainless steel, that produces a strong bond without braze erosion or the other problems described above. There is a need for a low temperature (<700° C.) brazing technique to avoid damage from creep deformation and thermal expansion mismatch between the foam and the stainless steel.